Vehicles such as C-130 and KC-135 military transport aircraft frequently travel in formation. The C-130 is a medium-range, tactical airlift aircraft designed primarily for transporting personnel and cargo. Although the KC-135 is an aircraft dedicated to air refueling, it also flies command post and reconnaissance missions. However, other aircraft such as the B-52, the B-2, the B-1, the C-17, and the C-141 also fly in formation.
A typical in-trail position keeping formation is illustrated in FIG. 1. The formation consists of multiple three-aircraft cells. The wings follow their cell leader, while each of the cell leaders follows the leader of the cell that immediately precedes it in the formation. Although three cells are illustrated in FIG. 1, formations of thirty cells or more are possible. Also, other types of formations are possible. For example, a box formation is illustrated in FIG. 2, and a fluid trail formation is illustrated in FIG. 3. As shown in FIG. 2, the box formation is formed from two cells of aircraft flying in line-abreast formation. As shown in FIG. 3, in the fluid trail formation, the follower wing aircraft stay along a fixed radius of the leader, never passing forward of the leader, and avoiding the position directly behind the leader.
The spacing between formation members depends on whether the flight is proceeding under visual flight rules (VFR) or under instrument flight rules (IFR), on whether the formation is preparing to land, and on whether other mission parameters are controlling. Some formation geometries are only flown visually.
Also, there are two types of maneuvers that pilots perform during position keeping. The first type is a commanded maneuver involving, for example, speed, heading, and/or altitude changes to the aircraft trajectory as the aircraft performs position keeping missions. A drop command is a commanded maneuver to initiate drops of cargo or personnel. Commanded maneuvers can be either simultaneous or sequential, depending on the type of maneuver and mission requirement. For example, speed changes should always be simultaneous, while altitude changes can be either simultaneous or sequential. Both leader and follower aircraft perform commanded maneuvers.
The second type of maneuver is a position keeping maneuver. Follower aircraft perform position keeping maneuvers in order to maintain a desired relative position to the leader aircraft or other aircraft in the formation. Position keeping maneuvers can be performed alone, or in conjunction with commanded maneuvers. For example, during a commanded turn, the follower attempts to maintain the desired separation with the leader while simultaneously executing the commanded turn maneuver.
A typical formation flight might proceed with the aircraft initially taking off at fixed intervals and climbing to 2000 feet or the lowest altitude to clear the terrain. During this interval, each of the aircraft joins the formation. Following formation, the aircraft then climb at 180 knots to 20,000 feet. The aircraft cruise at 20,000 feet and at 190 knots making speed, heading, and altitude changes as required in order to reach a designated location such as a drop point. If the mission is to drop personnel or cargo, the aircraft typically slow to 125 knots for a personnel drop and 130 knots for a cargo drop and descend simultaneously to the drop altitude, which usually ranges from 700 feet to 1500 feet above the ground depending upon whether the drop is a personnel drop or a cargo drop. The exact altitude is selected based on the highest terrain within five miles of the formation centerline.
The lead aircraft provides its drift angle ten to fifteen minutes from the drop zone so that the follower aircraft can compensate for the difference between heading and track angle. (The drift angle is the angle between the aircraft's heading and the aircraft's actual ground track.) The drift angle is required so that the aircraft can fly the same ground track over the Earth. In order to compensate for drift when flying a ground track rather than a heading, the follower aircraft calculate the lateral offset required to compensate for the drift, and input this offset into the position keeping equipment.
Once the personnel or cargo has been dropped, the follower aircraft reset their lateral offset to the nominal value, all aircraft climb to 20,000 feet, and all aircraft accelerate to 180 knots. During such cruising, speed, heading, and altitude changes may be required during the return to base.
Prior to the final approach fix, the aircraft formation longitudinal spacing is changed to 6000 feet. The aircraft land about 60 seconds apart.
Some of the changes that can occur while the aircraft are en route include rerouting, a change in drop zone, a change in leader, and a change in formation as members leave the formation and others move up.
Current position keeping systems use radar to measure the actual relative positions in the formation and use this relative position measurement to maintain the desired relative positions in the formation. Such systems can be unreliable, expensive, and do not take advantage of data from on-board navigation systems for position keeping. Also, they do not use wind direction in maintaining position in the formation. Therefore, these systems cannot automatically fly a track instead of a heading. Also, current position keeping systems cannot combine some maneuvers, such as altitude changes and turns.
Furthermore, current position keeping systems do not permit lead aircraft to perform formal position keeping maneuvers. Lead aircraft of one cell may informally track the leader of another cell, but they do not receive either formal guidance on their position keeping displays or commanded maneuvers from the other aircraft. The present invention is not so limited and allows lead aircraft in a cell to follow the lead aircraft in another cell.
The present invention overcomes one or more of these or other problems.